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Basically we agree with FSF on this issue. Quoting the GPL FAQ:
A: Using the GNU GPL will require that all the released improved versions be free software. This means you can avoid the risk of having to compete with a proprietary modified version of your own work.
We don’t want people to take PJSIP, mess it up (erm, improve it), and keep the improvements as proprietary code. On the contrary, we want everybody to enjoy PJSIP and all its improvements, and the only way to make sure of this is by releasing PJSIP as GPL. Sure this still leaves some debate over why not use, say LGPL, but I guess this page is probably too short to cover that. Our stand on LGPL is explained much better here.
It depends. We use the standard GPL v2 or later for PJSIP, and GPL does allow using GPL-ed code for closed source development, as long as the resulting product is not redistributed (for example, it is only used for internal purpose). Please see GPL FAQ for more information about what can/can’t be done with GPL software.
Alternatively, PJSIP can be used with Alternative license.